(a) Technical Field
The present disclosure relates to a method and system for controlling braking force in regenerative brake cooperative control, and more particularly, to a method and system for controlling braking force in regenerative brake cooperative control of an environmentally friendly vehicle which executes regenerative braking at front wheels and/or rear wheels.
(b) Background Art
In general, regenerative brake cooperative control of an environmentally friendly vehicle (e.g., a hybrid vehicle, an electric vehicle or a fuel cell vehicle) in which regenerative braking is executed at both front and rear wheels differs from regenerative brake cooperative control of a conventional vehicle in which regenerative braking is executed only at front wheels.
In an environmentally friendly vehicle in which regenerative braking of only front wheels is executed, driving motors are disposed at the front wheels. Regenerative braking force is generated when the driving motor charges a battery to recover energy and such braking force acts on only the front wheels. Since a possibility of generating the spin of the vehicle is low even when the overall braking force of the front wheels is substantial due to the regenerative braking force of the front wheels, to recover the maximum amount of energy, the generation amount of regenerative braking force may be maximally increased. Further, a system for cooperative control of hydraulic braking force is configured in consideration of only regenerative braking force of the front wheels.
For an environmentally friendly vehicle in which regenerative braking may be executed at both front wheels and rear wheels, regenerating braking force is applied to the rear wheels as well as the front wheels. When regenerative braking force of the rear wheels is increased to recover a substantial amount of energy, the rear wheels are locked and a possibility of generating the spin of the vehicle is increased and, thus, an increase in regenerative braking force of such an environmentally friendly vehicle as much as in the vehicle in which regenerative braking is executed only at the front wheels is limited.
FIG. 1 is a graph illustrating braking force distribution of a conventional vehicle in which regenerative braking is executed only at front wheels according to the related art, and FIG. 2 is a graph illustrating a relationship between actual front and rear wheel braking force distribution and an ideal braking force distribution line (e.g., an ideal braking distribution line) in the vehicle in which regenerative braking is executed only at front wheels according to the related art.
As exemplarily shown in FIG. 1, regenerative braking force of the front wheels is first used to increase the recovery amount of energy and, when a braking force that exceeds front wheel regenerative braking force is necessary, hydraulic braking force is generated by applying the same hydraulic pressure to wheel brakes of the front and rear wheels.
For front wheel regenerative braking, in spite of such braking force distribution, a deceleration at which lock of the rear wheels first occurs is greater than that of a vehicle in which only a general hydraulic brake is applied, as exemplarily shown in FIG. 2, thereby not greatly reducing vehicle stability. In other words, with reference to the actual braking distribution lines, only front wheel regenerative braking force is generated up to the maximum value of front wheel regenerative braking force and, thereafter, braking forces of the front and rear wheels by the hydraulic brake is generated. This is due to a cross point with the ideal braking distribution line being formed in a deceleration region where braking force is relatively large.
The magnitude of regenerative braking force generated from the front wheels is proportional to the capacity of the driving motors, and a deceleration at which lock of the rear wheels may first occur is changed based on the capacity of the driving motors. Therefore, in an environmentally friendly vehicle in which regenerative braking of only front wheels is executed, although braking force of the front wheels is increased by front wheel regenerative braking force, vehicle stability is not reduced. Therefore, a braking force ratio of front and rear wheel hydraulic brakes may not be changed and an X-split pipe line generating the same hydraulic pressure at the front and rear wheels may be used as the pipe line of a brake system.
Furthermore, FIGS. 3 and 4 are graphs illustrating braking force distribution according to the related art where the conventional brake system and braking force distribution are applied to an environmentally friendly vehicle in which regenerative braking of rear wheels or both front and rear wheels is executed, and actual braking force distribution.
As exemplarily shown in FIGS. 3 and 4, when an environmentally friendly vehicle in which regenerative braking of rear wheels or both front and rear wheels is executed uses the above-described conventional brake system and braking force distribution, regenerative braking force of rear wheels is first used to increase the recovery amount of energy and, when a braking force that exceeds rear wheel regenerative braking force is necessary (e.g., when a deceleration equal to or greater than “A” is required), hydraulic braking force is generated by applying the same hydraulic pressure to wheel brakes of the front and rear wheels.
In particular, when regenerative braking force is maximally used to recover energy, a deceleration having a possibility of occurrence of earlier lock of the rear wheels is reduced and thus vehicle stability deteriorates and, when regenerative braking force is restricted to assure vehicle stability, the recovery amount of energy is reduced. Therefore, a regenerative brake cooperative control method which may enhance stability and braking performance of a vehicle and maximize regenerative braking to improve fuel efficiency is required.